Image: New Class of Cross-Shaped Gravitational Lenses for Probing Structure of the Cosmos

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These two objects represent a new distant class of quadruple, or
cross-shaped, gravitational lenses which might eventually provide
astronomers with a powerful new "magnifying glass" for probing a
variety of characteristics of the universe.

The two gravitational lenses were discovered in about 100 fields of sky
imaged by Hubble Space Telescope's Wide Field Planetary Camera 2. The
first cross-shaped lens was discovered serendipitously by Eric
Ostrander while processing HST images for the Medium Deep Survey, a
Hubble key project led by Richard Griffiths. A second fainter and
smaller lens was identified a few weeks later by Myungshin Im. Each
configuration is in the form of four faint blue images situated
symmetrically around a much brighter red elliptical galaxy. The
distinctive cross-like pattern around an elliptical galaxy makes them
unambiguous quadruple lens candidates, even before spectroscopic
observations, which are typically used to confirm lenses.

Hubble's high resolution allows astronomers to extend the search to
much fainter, and hence much farther lenses, than those few examples
ground-based telescopes have uncovered relatively nearby. Hubble can
explore a larger volume of space which could provide enough examples of
this rare cross type of lensing to allow astronomers to address a
variety of fundamental cosmological questions.

A gravitational lens is produced by the enormous gravitational field of
a massive object which bends light to magnify, brighten and distort the
image of a more distant object. Depending on the alignment between the
objects and the mass distribution of the foreground lens, the more
distant object can be smeared into arcs or split into pairs, triples,
or even quadruple images.